In seismic surveying, two types of acoustic detecting transducers are presently used: geophones on land and hydrophones in marine conditions and in mud-filled bore holes. Pressure sensitive phones are ideally suited for marine work as pressure changes above or below ambient water pressure are recorded. On land, pressure sensitive phones can not be used as it is generally impractical to bury the phones in such a way that they would have adequate fluid coupling with the surrounding material.
Acoustic energy generates signals and is normally created by explosives, mechanical wave generators provided by truck mounted vibrators, or rams engaging the ground surface. A geophone is designed to measure particle velocity by conversion to electrical energy. Although this specification will be described by reference to geophones as the preferred sensor for a land-based seismic data acquisition system, other sensors requiring protection against lightning strikes are part of the system to be protected of this invention.
It is important that good coupling be obtained between the ground surface and the geophones. The geophone case has a spike or other base attached to the geophone case for implantation within the earth's surface. The case encloses a magnet and surrounding coil. The coil may be fixed and the magnet suspended, or the coil may be suspended and the magnet fixed. Relative movement between the coil and the magnetic field of the magnet induces an electrical voltage which is transmitted and measured in a recording system. Geophones are usually deployed in groups at each detector location with series connected geophone groups connected in parallel to form a string. Each string is attached to a takeout in the main cable which feeds to a specific channel in the recording system. Frequently, several cables are deployed to adequately sample the signals from the earth. Thus, seismic signals are detected by arrays or single geophones, and field equipment is normally provided to record the reflected seismic signals. The field equipment typically includes remote acquisition electronics, telemetry and central control electronics for control, display and storage.
Three dimensional (3D) seismic data is collected by deploying multiple lines of sensors over a large area of the earth. The seismic reflections to points on the surface form a three dimensional array of data which enables a geophysicist or geologist to process such data to produce various displays of the sedimentary structures in the earth.
Large surface arrays to collect 3D seismic data may require many square miles of cables and geophones. The many miles of cables, such as, for example, fifty (50) miles of multi-conductor cable with geophone strings spread over ten (10) to fifteen (15) square miles, is quite vulnerable to lightning strikes. A direct lightning strike on a cable, a geophone string, or acquisition box will cause damage at the point of entry. The surge of electricity may travel along the cables to other parts of the array, damaging cables, sensors such as geophones and other equipment as it passes. Thus, it is desirable to minimize damage from any lightning strikes by limiting the propagation of current from a strike location to other parts of the system.